The invention relates to a method and a device for the production of NaDCl4, in which D is aluminium or iron, where, in a first reaction step, a melt of aluminium or iron is reacted with chlorine gas to give gaseous metal halide, and this is subsequently reacted, in a second reaction step, with solid sodium chloride to give the corresponding compound and is separated off as a melt
Melts of salts, such as, for example, NaAlCl4 have various areas of application. Salt melts can be employed as storage medium in heat storage systems, as heat transfer media, for example in heating baths, for covering and cleaning molten metals, for the electrocoating of high-melting materials or as melt electrolytes in primary batteries, as described in GB 2046506. A further potential application of these salts is in rechargeable sodium batteries. The salts are employed in batteries which have operating temperatures of between 130xc2x0 C. and 200xc2x0 C. (Abraham. J. Electrochem. Soc., Vol. 137, 1189-1190, (1990)).
DE 3419279 describes an electrochemical cell in which the cathode matrix is impregnated with a sodium aluminium halide salt melt electrolyte.
A relatively new area of application is the xe2x80x9cZEBRA batteryxe2x80x9d. This high-temperature cell consists of an electrode of liquid sodium, a beta aluminium electrolyte and an electrode of transition-metal chloride in an NaAlCl4 melt (Cleaver, J. Electrochem. Soc., Vol. 142, 3409-3413, (1995)).
DE 3718920 describes the production of salt melts by the addition of a pure metal and an alkali metal halide to the melt The reaction cell is operated at above the melting point of the salt melt. The alkali metal halide in the working example is NaCl, the molten alkali metal is sodium, and the separator is beta-aluminium oxide. Owing to the use of pure sodium, special safety precautions, such as working under a protective-gas atmosphere, have to be taken. The reactions must take place in separate cells, since poisoning of the separator by the by-product AlHal3 formed must be prevented.
All the methods known hitherto for the production of salt melts work batchwise. A batch procedure has some severe disadvantages compared with a continuous production method. In the case of a batch change, the apparatus has to be opened. The product may then be contaminated by the oxygen from the ambient air, water and dust The batch change results in down times of the plant and thus in a reduced space-time yield. For an effective discontinuous method, large apparatuses have to be used. The start-up process requires correspondingly more energy and time. It has been found that impurities are entrained into the process, in particular during start-up of plants. FR 2168912 describes a complex purification method for alkali metal aluminium halides. The two-step purification process is composed of an oxygen treatment for degradation of the organic impurities and an aluminium treatment for precipitation of iron and heavy metals. The aluminium treatment must be carried out under a nitrogen or argon atmosphere.
For the production of the alkali metal aluminium halides, the reaction of corresponding aluminium halides and alkali metal halides in a closed tube is described (Friedmann, J. Am. Chem. Soc., 72, 2236-2243, (1950)). A pressure increase to 6-7 atmospheres has been observed in this method, which results in problems (FR 2168912). The apparatuses have to be fitted with the appropriate safety precautions.
The object of the invention is to provide a continuous method for the production of pure salt melts which excludes disadvantageous ambient influences, minimises the energy demand and facilitates an optimum space-time yield.
A further object is to make large quantities of salt melts available in the shortest possible time.
The object according to the invention is achieved by a method for the production of salt melts, and mixtures thereof, of the general formula
NaDCl4 xe2x80x83xe2x80x83(1)
in which
D is Al or Fe,
which, in a first reaction step (i), a melt of aluminium or iron is reacted with chlorine gas to give gaseous metal halide (DCl3), and this is subsequently, in a second reaction step (ii), reacted with solid sodium chloride to give the corresponding compound of the formula (I) and is separated off as a melt.
The invention furthermore relates to a device for carrying out the method, essentially consisting of a reaction vessel (1) containing the melt of the metal D, with a feed device for chlorine gas (2), a collection device for gaseous metal chloride (4) above the reaction vessel (1), and a further reactor vessel (5) which contains sodium chloride in solid form and is connected to the said collection device.
The products from the method are suitable for use as melt electrolyte in electrochemical cells, as storage medium in heat storage systems, as heat transfer medium, for example in heating baths, for covering and cleaning molten metals, for the electrocoating of high-melting materials or as melt electrolytes in rechargeable sodium batteries and primary batteries.
In the alternative methods, the solids, for example NaCl and AlCl3, are mixed and warmed to the melting point. The amount of heat necessary for this purpose has to be supplied from the outside.
Surprisingly, it has been found that the exothermicity of the reaction of aluminium or iron (D) with Cl2 to give aluminium chloride or iron chloride (DCl3) can be utilised for the further process for the production of NaDCl4.
In the process according to the invention, DCl3 (where D=Al or Fe) is formed at temperatures between 700xc2x0 C. and 1200xc2x0 C. In contrast to conventional methods, this DCl3 is fed in gaseous form to an alkali metal salt bed of NaCl.
It has been found that the accompanying heat of the gas (DCl3) is sufficient to warm the alkali metal salt (NaCl) to the melting point of the salt melt NaDCl4 (I).
An essential advantage of the method is the use of cheaper raw materials and the utilisation of the heat of reaction being liberated for controlling the temperature of the method. Method steps, such as the condensation of the metal halide (DCl3), can thereby be saved and the energy demand for carrying out the process reduced.
It has been found that the continuous performance of the method enables interfering environmental influences to be excluded. This enables a constantly high quality of the product to be established after the start-up phase.
All continuously operating reaction vessels which appear suitable to the person skilled in the art can be used for the method. For the reaction with chlorine gas, a feed device with gas inlet is necessary. The reaction vessel is provided with a refractory lining. A ceramic lining which is insensitive to the materials employed and the high temperatures is advisable.
The metal D in powder or granule form is provided for the process via a solids metering unit (3).
For collecting the reaction product (DCl3), a collection device (4), provided with a feed line to the downstream reaction vessel (5), is installed above the reaction vessel (1) for the melt
The feed of the reaction product takes place between the upper quarter and the lower quarter of the reactor vessel (5), which contains a mixture of metal D and sodium chloride in solid form. Complete conversion of the reactants into the reaction products can thus be ensured.
The salt melt formed runs off in a downward direction through the alkali metal salt bed (NaCl), which is supported by a support grille or a coarse filter plate.
A mixture of solid sodium chloride and metal D in powder or granule form is fed continuously, corresponding to the amount of end product formed and separated off, to the reaction vessel (5) via a solids metering unit (6).
Due to water entrained by the raw materials, undesired HCl gas forms. This can react away to give the metal halide (DCl3) due to amounts of the corresponding metal granules or powder (D) present in the alkali metal salt bed.
It is advisable for a further reactor vessel (7) with alkali metal salt bed to be installed downstream of the reactor vessel (5) in the flow direction for purification of the melt in order to allow the metal halide (DCl3) formed in turn to react away to give NaDCl4.
A temperature control device is merely necessary for heating up once in the start phase and, where appropriate, for cooling. The energy necessary for melting the metal granules (D) is also provided by the heat of reaction.
The method can be carried out continuously or discontinuously as required.
A general example of the invention, which is shown in the drawing, is explained in greater detail below. FIG. 1 shows a reaction vessel containing metal melt 1 having a feed device for chlorine gas 2 and solids metering device 3, collection device for gaseous metal chloride DCl3 4 and reactor vessel 5 containing metal granule or metal powder and alkali metal salt bed, and solids metering device 6, as well as a downstream reactor vessel 7.
Reaction Step i:
For the production of salts conforming to the formula (1) and mixtures thereof, the raw materials can be fed to the reaction vessel (1) in premixed form via the solids metering device (3). The filling can be carried out under inert gas.
The heatable reaction vessel (1) contains liquid metal melt. Suitable metals (D) are iron and aluminium. Chlorine gas is fed into the reaction apparatus via the feed device (2). The volume of the melt and the volume flow of the gas is determined as a function of the requisite residence time and the desired throughput. A temperature above the melting point of the metal (D) is set in the reaction vessel (1).
The gaseous metal halide (DCl3) is fed to the reaction vessel (5) via the collection device (4) for the reaction product formed.
Reaction Step ii:
The metal halide is fed to the metal granule or metal powder and alkali metal salt bed between the upper and lower quarter of the reactor vessel (5), preferably between the upper quarter and the centre. A mixture of metal granules or powder (D) and alkali metal salt (NaCl) is fed constantly to the reactor vessel via a solids metering device (6) in accordance with consumption.
The metal halide (DCl3) is reacted with the alkali metal salt in the reactor vessel (5) to give NaDCl4.
Reaction Step iii:
The melt maybe contaminated due to contact with water or atmospheric moisture. The hydrogen halide formed can react away with the metal granules (D) added to the salt bed in the reactor vessel (5) to give the metal halide (DCl3).
Reaction Step iv:
For further processing, the metal halide is passed through the reactor vessel (7). Row takes place from bottom to top through the purification unit charged with alkali metal salt NaCl. In the process, the metal halide DCl3 is reacted with the alkali metal salt NaCl to give the desired salt NaDCl4.
The flow from bottom to top through the reactor vessel (7) is not absolutely necessary. However, it has the advantage that the particles becoming smaller due to the reaction are not forced onto the sieve plate by the flow, blocking it. Nevertheless, homogeneous through-flow (plug flow) in the column is ensured in this way. Homogeneous through-flow is an essential prerequisite for complete reaction in the purification unit.